Dispersion strengthened copper is now a relatively well known material which is particularly useful in the fabrication of electrodes for automatic resistance welding machines used, for example, in the manufacture of automobiles. Reference may be had to the patent to Nadkarni et al, U.S. Pat. No. 3,779,714 which discloses a method of dispersion strengthening copper by internal oxidation. U.S. Pat. No. 3,179,515 shows another method of internally oxidizing alloys by surface oxidizing a powdered alloy and then diffusing oxygen into the powder particles to preferentially oxidize a solute metal to solute metal oxide. British Pat. No. 654,962 shows a method of internally oxidizing silver, copper and/or nickel alloys containing solute metals by oxygen diffusion to increase the hardness of the alloy.
Heretofore, bar stock for the production of dispersion stengthened copper electrodes has been produced by a process for canning a dispersion strengthened copper powder, and then extruding through a die to produce a dispersion strengthened rod or bar. (See U.S. Pat. No. 3,884,676 to Nadkarni et al.) Reference may also be had to U.S. Pat. No. 4,045,644 to Shafer et al which shows a process for making a welding electrode from dispersion strengthened metal to improve the grain structure in the electrode tip portion and thereby improve the life of the product.
It has been found that extrusion of a "canned" dispersion strengthened copper powder results in the formation of a densified dispersion strengthened copper characterized by a grain structure in which the grains are substantially in alignment and have a fibrous nature. This is caused by high deformation ratios of the original cross-sectional area of can: cross-sectional area of extrudate used in the process of extrusion, e.g., from about 8:1 to about 200:1. As pointed out in the aforementioned U.S. Pat. No. 4,045,644, an upsetting operation is utilized in the manufacture of resistance welding electrodes to disturb the axial alignment of the fibers and thereby minimize failure of the electrodes by cracking generally in an axial direction longitudinally between the fibers as a result of impact in use.
The present invention provides an improved process for densifying dispersion strengthened metal powder in a metallic sheath or container by staged size reduction in a plurality of stages, some or all of which may be carried out at elevated temperature, e.g., 1000.degree. F., or higher. Staged size reduction alone has been found to be insufficient to assure complete densification of powder and maximum electrode life unless a relationship between the cold worked tensile strength of the outer sheath and the final tensile strength of the substantially fully densified dispersion strengthened metal is observed. "Staged size reduction" as used herein contemplates relatively small size reduction per pass, such reduction being in the range of from about 15% to 35% of the cross-sectional area of the workpiece until at least about 90% of theoretical density, and preferably full density has been achieved. Size reduction may be accomplished by applying compressive force continuously during a given pass, as with rolling, or intermittently during a given pass as with swaging. Usually extrusion is done with very much larger size reduction, i.e., of the order of from about 80% to 99% per pass (See U.S. Pat. No.: 3,884,676). Size reduction of this magnitude with containerized dispersion strengthened powder requires a large investment capital in extrusion apparatus. The present process is less costly from the standpoint of investment and cost of operation. Hence, products can be produced at reduced cost.
Staged size reduction is carried out preferably until full density is achieved. Even during staged size reduction, it has been found if these relative tensile strengths are too disparate, relative deformation in an axial direction between the outer sheath and the inner core is experienced to an extent sufficient to cause cracking of the core. It has been found, therefore, that the cold worked tensile strength of the sheath should not be less than the tensile strength of the fully densified core by more than about 22% to 25% of the ultimate tensile strength of the core. In the case of dispersion strengthened copper, this difference is about 15,000 psi.
The improved process utilizing a swaging machine or rod rolling, involves a lower capital expenditure initially and a lower labor content than the previously practiced extrusion method.